1. Field of the Invention
This invention relates to an evaporative fuel-purging control system for internal combustion engines equipped with evaporative emission control systems.
2. Prior Art
Conventionally, evaporative emission control systems have widely been used in internal combustion engines, which operate to prevent evaporative fuel (fuel vapor) from being emitted from a fuel tank into the atmosphere, by temporarily storing evaporative fuel from the fuel tank in a canister, and purging same into the intake system of the engine. Purging of evaporative fuel into the intake system causes enriching of an air-fuel mixture supplied to the engine, so that the air-fuel ratio of the mixture deviates from a desired value. To overcome this disadvantage, an evaporative fuel-purging control system has already been proposed in U.S. Ser. No. 07/853,288 by the present assignee, in which a mass flowmeter is inserted in a purging passage extending between the canister and the intake system for measuring the flow rate (purging flow rate) of a mixture of air and the evaporative fuel flowing through the purging passage, to correct the air-fuel ratio in dependence on the measured flow rate.
According to the proposed system, the opening of a purge control valve, which is provided across the purging passage for controlling the flow rate of the mixture in the purging passage, is controlled so as to make the detected output value from the mass flowmeter equal to a desired value.
In general, the purging flow rate varies greatly between a very small value and a very large value, depending on the amount of evaporative fuel stored in the canister and negative pressure within the intake system acting to draw the mixture from the purging passage into the intake system. Further, the concentration of evaporative fuel in the mixture flowing through the purging passage largely affects the air-fuel ratio of the whole mixture supplied to the engine. Particularly when the engine is operating under a low load condition, the amount and/or concentration of evaporative fuel purged has a great influence on the air-fuel ratio due to a reduced amount of fuel injected into the engine.
However, in the proposed system, the zero point correction of the output from the flowmeter is not carried out, so that a large error can occur in the output value from the flowmeter, i.e. the measured flow rate of the purged mixture particularly when the purging flow rate is small (i.e. under a low load operating condition of the engine), which brings about a problem of insufficient accuracy of control of the air-fuel ratio by the use of the purge control valve.
On the other hand, the purge control valve may be a duty control type as disclosed e.g. in Japanese Provisional Patent Publication (Kokai) No. 62-20669. In such a case, the purge control valve is substantially closed when the duty ratio of the valve is in a very small value region, so that the purging flow rate is substantially reduced to the minimum value. In view of this fact, in the disclosed system, a maximum value of the duty ratio at which the purging flow rate is maintained at the minimum value is checked and stored beforehand, and a duty ratio corresponding to a desired purging flow rate is calculated by the use of the stored maximum value of the duty ratio as a control zero point, whereupon a signal indicative of the calculated duty ratio is supplied to the purge control valve.
However, the aforementioned maximum of the duty ratio at which the purging flow rate is maintained at the minimum value varies from one purge control valve to another, and also changes with aging of the valve. Therefore, if the control zero point is set to the same or fixed value for all the purge control valves or irrespective of aging of the valve, it is impossible to obtain the same purging flow rate even if a signal indicative of the same duty ratio is supplied to the purge control valves, i.e. there can arise an error in the purging flow rate.